This invention relates to a proximity detector having a latching ferro-magnetic proximity detector including a Hall element, a Schmitt trigger circuit and a voltage regulator which may perform abnormally producing at the output of the Schmitt trigger circuit spurious binary signals during periods when either an over voltage or under voltage appears on the DC voltage supply line, and more particularly pertains to such a detector that includes a logic circuit for blanking and preventing anomalous binary signals or "noise" pulses from propagating through from the DC voltage supply line to the output of the magnetic field detector.
It is well known to make integrated circuit magnetic field sensors and proximity detectors that include a Hall element, serving as a magnetic-field-to voltage transducer, followed by a Schmitt trigger circuit. Various of such known sensors are described in the U.S. Pat. No. 4,374,333 to G. Avery; U.S. Pat. No. 4,443,716 to G. Avery; U.S. Pat. No. 4,518,918 to G. Avery; U.S. Pat. No. 4,705,964 to J. Higgs; U.S. Pat. No. 4,761,569 to J. Higgs; U.S. Pat. No 4,791,311 to R. Vig, D. Demingware and A. Tu; and U.S. Pat. No. 5,045,920 to R. Vig; these patents being assigned to the same assignee as is the present invention.
Typically the binary output signal of the Schmitt trigger circuit goes high when the ambient magnetic field from the South end of a magnet is rising and exceeds a predetermined polarity and magnitude, that predetermined magnitude, B.sub.op, being established by the characteristic upper threshold of the Schmitt trigger circuit. Schmitt trigger circuits have hysteresis, so that subsequently, when the magnetic field falls to a magnitude, B.sub.rp, established by the characteristic lower threshold of the Schmitt trigger circuit, the binary output voltage changes to a low value.
These magnetic-field detector circuits are usually formed in a silicon integrated circuit that may include a simple voltage regulator that provides a regulated source of DC voltage, V.sub.REG, for energizing the Hall element and the Schmitt trigger circuit.
We have determined that in these prior art magnetic-field detector circuits, when a large negative noise spike appears on the DC voltage line that is connected to and energizes the voltage regulator, and at the same time an ambient south-pole magnetic field strength at the Hall element exceeds B.sub.op, the regulated DC voltage drops and the output voltage of the Schmitt trigger circuit will momentarily change from a binary high to a binary low. In other words, an anomalous Schmitt circuit output pulse will be produced that is not a response to a change in magnetic field. Thus in response to a burst of negative noise pulses during a period when the field exceeds B.sub.op, the detector will produce multiple false signals.
In such conventional detector circuits, negative noise pulses do not cause anomalous output signals during periods when there is a low south going ambient magnetic field and the resulting output of the Schmitt circuit is already low, because each large negative spike on the DC supply line to the regulator causes the output of the regulator to drop to near zero and produce a binary-low Schmitt-circuit-output signal. This binary-low level corresponds to the signal which already exists, being attributable to a low ambient magnetic field (less than B.sub.op) at the Hall element.
Positive noise pulses likewise cause no anomalous output signals during periods when there is a low south going ambient magnetic field (less than B.sub.op) and the output of the Schmitt circuit is therefore low for the same reason.
If the voltage regulator includes an over-voltage protection circuit whereby the regulator circuit is caused to shut down during the presence of a positive, i.e. an over-voltage noise spike, the regulated voltage itself, V.sub.reg, momentarily drops to near zero in response to a such a positive noise spike. Such an over-voltage protection circuit is described in the patent to Higgs, Kawaji and Vig, U.S. Pat. No. 4,751,463 that is assigned to the same assignee as is the present invention.
Thus in such a detector, when either a large positive or negative noise spike appears on the DC voltage line during a period when the magnitude and polarity of the magnetic field are such that B.sub.op is exceeded, the regulated DC voltage rises and the output voltage of the Schmitt trigger circuit will have momentarily lost power and will momentarily change from a binary high to a binary low. In other words, an anomalous Schmitt circuit output pulse will be produced that is not a response to a change in magnetic field.
It follows that, in a conventional magnetic field detector with a regulator having an over-voltage protection circuit, the Schmitt circuit responds in the same way to both positive and negative noise (voltage out of range) on the Vcc line, for the condition that the noise occurs during a period when the magnetic field exceeds B.sub.op. However, for the case when the magnetic field is less than B.sub.rp, then the Schmitt circuit output signal is low or near zero and neither polarity of noise or out-of range DC supply voltage results in false outputs.
We have also noted that when the DC supply voltage from which a conventional magnetic-field detector is energized comes from a voltage regulator, that regulator and its output regulated supply voltage, Vreg, tend to perform erratically when the DC supply voltage, Vcc, from which the regulator is energized swings outside a predetermined voltage band; and the ultimate result tends to be the generation of false binary signals from the output of the magnetic-field detector, i.e. signals that are irregularly responsive to changes in the ambient magnetic field. Further the insertion of a voltage regulator between a source of DC supply voltage, Vcc, and the DC supply conductor of a conventional magnetic-field detector tends to exaggerate the deleterious effect on accurate performance of a magnetic-field detector of spurious noise spikes superimposed on the DC supply voltage, Vcc.
It is therefore an object of this invention to provide a magnetic field detector that essentially blanks DC-line noise pulses from feeding through to the detector output during periods when the ambient magnetic field is of a strength and polarity to render the binary output of the Schmitt trigger circuit high.